The present invention relates to a method and a device for crushing of material in a crushing plant having multiple crushing stations.
Methods and devices of the types stated above have been used for a long time to optimise crushing in crushing plants. In crushing plants, such as in mines and ballast plants, use is made of plurality of crushing steps to obtain a desired degree of reduction of the crushed material. The primary crushing step usually is a jaw crusher or a spindle crusher and can be supplied with a cubic meter large blocks of material. Secondary, tertiary and quaternary crushing steps usually comprise cone crushers but can also be impact breakers or mill grinders, and perform crushing of finer materials. The number of crushing steps varies according to the desired reduction of material, the so-called degree of reduction, and also how difficult it is to crush the material. In each crushing step, a plurality of crushers of varying size can be arranged. An important functional consideration when operating crushing plants is that the different crushing steps are balanced, i.e. that the crushers are subjected to a uniform load in the different crushing steps. It is of great economic importance that the crushing plants are operated without unnecessary stoppages.
When installing a crushing plant, the manufacturer of the crusher usually performs a dimensioning of equipment, such as crushers, screens, feeders and conveyors, to obtain a uniform load in the plant. During operation of the plant, variations in the production capacity, however, will arise, inter alia owing to wear on equipment and variations in the properties of the crushed material. When imbalance between two different crushing steps in the crushing plant occurs, it has been solved by turning off the crushing in one crushing step or by alternating the crushing in the different crushing steps. This has resulted in great losses of capacity of the plant and, consequently, reduced efficiency. Moreover, the uneven operation of the crushers in the different crushing steps has caused uneven wear between the different crushing steps. This has in turn resulted in more service occasions, which has caused a great consumption of time and great expenses for repair and maintenance work.
Another way of balancing crushing capacities in two subsequent crushing steps using gyratory crushers has been to change the eccentric motion of the different crushers, also referred to as stroke. By changing the stroke in a crusher, there arises a greater or smaller difference between the maximum and the minimum crushing gap in the crushing chamber of the crusher. The crushing gap is the distance between the crushing surfaces in the crushing chamber where the crushing is carried out. In case of a larger stroke, an increase in the capacity of letting through crushed material arises in the crusher, and in case of a smaller stroke a corresponding decrease arises. In this manner, one has roughly tried to balance the flow in crushing plants. Unfortunately, such adjustments of the stroke are time-consuming since the crusher must be dismantled to enable a change of the setting of the eccentric bushing in the crusher. Therefore, the stroke in the crushers is rarely changed although imbalance has occurred between different crushing steps. Instead the operator usually stops the supply of material to the crushers in the different crushing steps when imbalance arises between the crushing steps.
A further way of adjusting the capacity of letting through material in certain crushers is to change the smallest crushing gap, Closed Side Setting (CSS). This can be carried out, for example, by changing the distance between the crushing surfaces (inner and outer shell) in the crushing chamber. There are crushers in which the gap is changed by raising or lowering the outer shell of the crusher. This is achieved by turning the upper part of the crusher, which according to requirements of manufacture is allowed to take place only once an hour. Other crushers are available, in which the gap is changed by hydraulically raising or lowering the inner shell of the crusher. As a rule, the crushers are operated with a gap which results in a desired crushed product, such as maximum reduction or optimum grain form. By grain form is meant the degree of cubic form of the material.
In traditional crushing, the crushers in each crushing step are operated with a suitable stroke and gap. The different crushing steps in the crushing plant are adapted to the initial circumstances. However, since there are considerable differences in the properties of material during crushing and the outcome varies as crushing surfaces are being worn, imbalance arises in the plant. When the intermediate storage between two crushing steps has become too large or too small, the crushing in one of the crushing steps has been turned off. When a normal level of material in the intermediate storage has then been achieved, the crushing steps are again started and operated simultaneously. Level monitors are used to monitor the level of material in material storages or material compartments before the different crushing steps. Signals from the level monitors are transmitted to control units which control the supply of material to the crushing steps.
Since crushers in different crushing steps can have capacities of up to several hundred tonnes/h, every small increase of the crushing capacity causes an increased production capacity.
An object of the present invention is to provide a method and a device for improving the crushing of material in a crushing plant which comprises at least two crushing stations.
A further object of the present invention is to obviate the above problems in prior-art technique.
One more object of the present invention is to provide an improved crushed product from crushing stations in crushing plants.
These and other objects that will be evident from the following description are achieved by a method and a device of the type described hereinafter.
Crushing is carried out by crushing material at the first crushing station and conveying at least those parts of the crushed product whose size exceeds the stipulated maximum grain size to the intermediate storage. The remaining parts of the crushed product are conveyed to a material outlet. The amount of material in the intermediate storage is monitored and the degree of reduction at the first crushing station is increased if the level of material in the intermediate storage exceeds a first predetermined level. If the level of material in the intermediate storage falls below a second predetermined level, the degree of reduction of the first crushing station is decreased.
The crushing means, for example, that the first crushing station must work harder with an increased degree of reduction and a lower capacity when the second crushing station does not manage to keep up. This results in a smaller number of stoppages in the crushing plant, which leads to an improved crushing economy. The crushing work of the crushing stations can be finely adjusted and thus be adapted to variations in material and wear based on the level of material in the intermediate storage.
By monitoring the level of material in the intermediate storage by means of a level monitor, the plant and, thus, the operation of the first crushing station can advantageously be automated. The degree of reduction of the first crusher will then be controlled with improved accuracy in respect of changes in material properties and the like, which causes an increased crushing efficiency.
Preferably the degree of reduction at the second crushing station is decreased if the level of material in the intermediate layer exceeds a first predetermined level. Correspondingly, the degree of reduction at the second crushing station is increased if the level of material in the intermediate storage falls below a second predetermined level. As a result, each crushing station will be utilised maximally in cooperation with the preceding or subsequent crushing station. The interplay between the different crushing stations makes it possible for the crushing stations to crush material essentially continuously without interruption, thus causing a greater utilisation of the capacity of the crushing stations.
When, for example, the level of material in the intermediate storage is too high, the degree of reduction at the first crushing station is increased. Then the total capacity through the first crushing station decreases while the amount of fine material of the crushed product, which passes the intermediate storage and the second crushing station to the material outlet, increases. With a large amount of fine material produced at the first crushing station, a decrease of the degree of reduction at the second crushing station can be made without a significant change in the composition of material in the material outlet. By changing the degree of reduction at the two crushing stations simultaneously, the level of material in the intermediate storage will be quickly restored.
Moreover, the change of the degree of reduction in the first crushing step preferably occurs at intervals of up to about 10 min, preferably up to about 5 min and most advantageously about 1 min. Corresponding changes can also be made for the second crushing step. This means that the degree of reduction of the crushing stations can be balanced continuously after changes in the levels that arise in the intermediate storage. This also results in the balance between the two crushing stations being rapidly restored in case of imbalance.
According to a preferred embodiment, a change in the degree of reduction at the first crushing station is achieved by changing the minimum crushing gap. Since the change of the gap can be carried out without a crusher at the first crushing station needing be dismantled, work and time will be saved.
The degree of reduction can advantageously be changed in operation to eliminate unnecessary stoppages. By operation is meant, for example, that the crushing station carries out crushing work as the change in the degree of reduction is being made. Alternatively, the crusher operates without supply of material as the change in the degree of reduction is being made.
The device for crushing material in a crushing plant has, according to a preferred embodiment, a level monitor for monitoring the level of material in the intermediate storage and a control unit for controlling the degree of reduction at two crushing stations arranged on each side of the intermediate storage. The intermediate storage is preferably monitored continuously. This makes it possible to improve the utilisation of crushers in the different crushing steps and obtain a more even operation in the crushing plant.